A mixer truck is a vehicle that transports mortar or ready-mixed concrete or the like (hereinafter referred to as “fresh concrete”) from a fresh concrete factory to a construction site by loading it into a mixer drum that is carried on a frame such that it can freely rotate. In order to prevent the fresh concrete from decreasing in quality and hardening, the mixer truck normally rotates the mixer drum when transporting the fresh concrete. Thereby, the fresh concrete is agitated by a plurality of spiral blades disposed within the mixer drum. On the other hand, if the mixer truck rotates the mixer drum in reverse, the fresh concrete within the mixer drum is discharged. Once the mixer truck arrives at a concrete pouring site, the fresh concrete can be supplied to a pouring location by rotating the mixer drum in reverse.
In such a mixer truck, the mixer drum must be normally rotated constantly until the fresh concrete is discharged. An engine of the mixer truck is typically used as a drive source of the mixer drum. Specifically, a rotary motive force of the engine is transmitted to a hydraulic pump via a PTO (Power Take Off), and then working oil discharged from the hydraulic pump is supplied to drive a hydraulic motor. The mixer drum is then driven to rotate by the rotation of the hydraulic motor.
In such a mixer drum driving apparatus that drives the mixer drum to rotate using only the driving force from the engine, the number of rotations of the engine must be increased especially when the mixer drum is to be rotated at a high speed. When the number of rotations of the engine is increased, noise is generated and the amount of fuel consumption rises. In order to solve this problem, for example, JP2007-278430A proposes a mixer drum driving apparatus that drives a mixer drum to rotate by driving an auxiliary hydraulic pump using an electric motor instead of driving a hydraulic pump by the engine when the mixer truck is stopped.